CrOS Settings: Convert Crostini page to TS

Additionally loads HTML templates for custom elements at runtime.
Some files in the guest_os/ folder were updated for type checking
and extension.

Bug: chromium:1315757, chromium:1292025
Test: browser_tests --gtest-filter="OSSettingsCrostini*"
Test: browser_tests --gtest-filter="OSSettingsGuestOs*"
Change-Id: Ice9dd973bc2f4cc9a7d3607e189e3f0217c9adbb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3928219
Auto-Submit: Wes Okuhara <wesokuhara@google.com>
Reviewed-by: Fergus Dall <sidereal@google.com>
Commit-Queue: Wes Okuhara <wesokuhara@google.com>
Cr-Commit-Position: refs/heads/main@{#1060806}